'Thumbs.db' is a file used in Microsoft Windows XP that stores a cache for Windows Explorer's thumbnail view. Thumbs.db is saved in each directory (Windows folder) that contains pictures or photos.

Microsoft Windows XP, by default, shows the thumbnails view of certain folders, whether you want it to or not. Even if you choose Details (or some other view) as your default by going to Tools -> Folder Options -> View tab and clicking Apply to All Folders, Windows Explorer may still revert back to the Thumbnails view. It does this for any folder in which it finds the THUMBS.DB file.

Windows stores thumbnails of graphics or image files (such as JPEG, BMP, GIF, TIF, PDF and HTM files) in thumbs.db to act as a preview of the (first page of) the file. Each graphic file created in a directory is represented in this database file as a thumbnail in the format of a small JPEG file, regardless of the file’s original format. Each Windows folder with initiated thumbnail views (that is where you have viewed the folder and selected Thumbnails view or Filmstrip view) will have a thumbs.db file. Thumbs.db is effectively a database of all the graphics files in each Windows directory.

Thumbs.db files are hidden system files (protected operating system files) so it may not be obvious that they even exist.
Thumbs.db files can be a particular problem with Network Shares.

Even if you delete Thumbs.db, Windows repeatedly re-creates the file automatically.

Thumbs.db files are intended to speed up the browsing of graphics files in Windows Explorer. Unfortunately they can take up large amounts of storage space and slow down the systems' performance.

In a document management or imaging system, where many thousands of TIF or PDF scanned documents are stored, allowing the thumbs.db file to be created may slow down performance considerably. In scanning sub-systems each document scanned may be added to a thumbs.db file, reducing scanning speeds.
If the information is confidential, security may be breached by thumbnails of all graphics files (i.e. scanned documents) being held in the Thumbs.db database. Thumbs.db remains behind even if the graphics files are moved or deleted.

Here's how to stop Thumbs.db files from being created:

From My Computer or Windows Explorer, go to Tools -> Folder Options
(or open the Folder Options icon in the Control Panel).
Choose the View tab, and make sure the Do not cache thumbnails option is ticked (see below).
Click OK.

The more technical amongst you can also change this setting in the Windows XP Registry:

Open the Registry Editor (regedit.exe).
Navigate to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced.
Double-click the DisableThumbnailCache value, or go to Edit -> New -> DWORD value to create a new value by that name.
Enter 1 for its value.
Click Ok and close the Registry Editor.
You'll have to log off and then log back on for this to take effect.

Windows Media Center Edition also creates an ehthumbs.db which holds preview of video files.

In Windows Vista, Microsoft replaced thumbs.db files with a centralised thumbnail database "thumbcache_xxxx.db" located in:
\Users\[user name]\AppData\Local\Microsoft\Windows\Explorer.

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